What Is a Spectacle Blind?

Quick Answer

A spectacle blind is a figure-8-shaped steel plate consisting of two discs connected by a short bridge (web). One disc is solid (the blind/spade), the other has a bore matching the pipe ID (the spacer/ring). By rotating the assembly 180 degrees, operators can switch between open flow and full line isolation without removing the component from the piping.

How It Works

The spectacle blind installs between two flanges in a bolted joint. In the “open” position, the bored spacer ring sits between the flanges, allowing full flow. In the “closed” position, the assembly is rotated so the solid blind disc sits between the flanges, completely blocking flow. The unused disc hangs outside the flange pair, visible to operators.

Property	Detail
Standard	ASME B16.48
Size range	NPS 1/2 to NPS 24
Pressure classes	150 through 2500
Common materials	A105 (CS), A182 F316 (SS), A182 F11 (alloy)
Thickness	Same as a blind flange of the same NPS and class
Gaskets	Two gaskets required (one on each side of active disc)
Bolts	Standard stud bolts for the flange pair (longer to accommodate thickness)

Spectacle Blind vs Spade and Spacer

ASME B16.48 covers three related components:

Component	Description	Installation
Spectacle blind	Figure-8 (blind + spacer connected)	Permanently installed; rotated to change position
Spade (paddle blind)	Solid disc only	Inserted/removed between flanges for isolation
Spacer ring	Bored disc only	Replaces spade when flow must resume

Spectacle blinds are preferred when isolation is switched frequently (e.g., maintenance turnarounds). Spades and spacers are used when isolation changes are rare and the piping can be opened to insert or remove the disc.

Pro Tip

Spectacle blinds require extra flange spacing to accommodate the thickness of the assembly plus two gaskets. The piping isometric must account for this additional length. On existing piping, adding a spectacle blind may require cutting and re-welding a spool piece.

When to Use Spectacle Blinds

• Positive isolation for maintenance (more reliable than closing a valve—zero leakage)
• Equipment isolation during turnaround shutdowns
• Process unit separation: Isolating one unit while another operates
• Hazardous service: Where valve leakage could cause safety incidents
• Hydrostatic testing: Isolating test boundaries

Common Mistake

Forgetting that rotating a spectacle blind requires depressurizing the line, removing the bolts, and using a crane or chain block for large sizes. It is not a “quick isolation” tool—plan for downtime and have the correct gaskets ready before starting.

Handle and Identification

The protruding handle (the half hanging outside the flange pair) is a visual indicator. Operators can see at a glance whether the line is open or blinded. Many projects paint the blind disc a contrasting color (red) and the spacer ring a different color (green) for quick identification.

Read the full guide to flanges